In general, only a fraction of the originally present oil can be recovered by primary recovery methods when extracting oil from oil-bearing deposits. In this procedure, the oil flows to the surface on account of natural deposit pressure and is pumped to the surface of the earth from the bottom of the bore hole when the energy decreases.
A further increase in oil yield can be achieved by secondary measures. In the latter, water or gas is introduced under pressure by means of injection probes. The most frequently utilized method is so-called water flooding wherein either formation water, produced in a closed cycle, is reinjected or suitable flooding water is injected. In the latter case, care must be taken that the ions of the injected water are compatible with those of the formation water. Water flooding serves to supply energy to the reservoir as well as to control displacement of the oil toward the production probes. In order to cover a maximally large portion of the pore space (a volumetric throughflow efficiency degree is correspondingly defined), correspondingly suitable arrangements of injection and production probes must be selected; at the same time, very high water injection rates must be employed. It is desirable in many cases to inject the water into the aquifer, but in some instances the oil zone is likewise contacted to a large extent during this process. In the frequently utilized closed-cycle mode of operation, water flooding involves injection of water still containing residual oil since a quantitative separation of oil and water is only rarely accomplished in the separators.
However, whenever liquids immiscible with each other coexist in the pore space, capillary pressures occur. These are the higher, the higher the interfacial tensions between the two liquids and the smaller the pore diameters in the pore space. In the injection of water, i.e., during water flooding, these capillary pressures must be overcome by the injection pressure. Model calculations (cf. D. Balzer, Oil Gas 1 [1983]) show that the capillary pressures to be overcome are at an extremely high level, and consequently many of the narrower pores cannot be flooded. This finding is especially grave in its consequence in the direct vicinity of the injection probes, where large amounts of water must pass within a short period of time through a relatively small carrier surface.
A solution of this problem, namely raising the relative water permeability in the zones of the injection probes, must consist in greatly reducing the residual oil saturation in these zones. The depth of these regions need not be very great; about 3-20 m will be adequate in most cases.
This reduction of residual oil saturation in the injection probe zones should be successful, in accordance with the present state of the art of tertiary oil recovery, by using tensides in the form of microemulsions. In fact, U.S. Pat. Nos. 3,474,865, 3,467,188 and 3,718,187 disclose injection probe treatment methods wherein microemulsions or micellar dispersions are utilized, i.e., systems consisting of oil, aqueous solution, tenside, cotenside, and electrolytes. This solution to the problem, though, has the drawback that the amounts of tenside for the production of microemulsions are usually relatively high. Besides, the phase characteristic of a microemulsion depends in many cases on the tenside concentration, which latter decreases with increasing advancement of the tenside solution into the deposit due to adsorption processes. Correspondingly, the phase characteristic is altered from state III toward state II.sup.+ (cf. G. J. Hirasaki et al., SPE 8825 [1980]), leading in most instances to a strong rise in viscosity of the dispersion, the injectability being reduced thereby instead of being increased.